This invention relates generally to a system for detecting an object in a light reflecting medium using lidar.
Several techniques have evolved over the years for overcoming the problems associated with detecting targets in a light scattering medium. One technique utilizes a narrow beam from a pulsed laser, such as a doubled YAG, to scan the medium. Generally, the beam transmitter and the receiver aperture, which must be quite large to collect sufficient energy, are scanned together, using scanning mirrors or other devices such as prisms. The energy received from each pulse is detected with a photomultiplier, or similar quantum-limited device, and the resulting signal is amplified with a logarithmic response amplifier, digitized, and then processed. Because the pulses are short, typically 10 nanoseconds, the detection electronics must be very fast, digitizing at 200 MHz or faster. Since the pulse rate is low, the processing rates required to analyze the data from each pulse are within the state of the art.
Another technique is range gating, which utilizes a pulsed flood beam and a number of gated image intensifiers with charge-coupled devices (CCD""s). The intensifiers are gated on when the beam pulse reaches a specific depth. Typically, the gate is applied just as the pulse beam encounters the object so that the full reflected return is obtained. A second intensifier is gated on a little later to detect the shadow of the object. The image of the object is obtained by taking the difference of the two images which then eliminates the backscatter from the medium and enhances the object signature. This yields a horizontal slice, but not a vertical slice or third dimension.
Despite the availability of such techniques, existing lidar systems are limited by the size of the receiver optics that can be used in a scanner. Generally, the light reflected from objects that are deep within a medium or are in a very turbid medium is weak. Although large aperture optics can aid in maximizing the amount of light collected from weak returns, the size of the optics that can be used in a scanner is restricted by the size of the moving prisms or mirrors.
Furthermore, those systems which utilize range gating, instead of volume scanning, suffer from poor range resolution and area coverage. When the object is at a different depth than expected, the object return will be subtracted as well as the background, and poor performance results. Additionally, very large pulse energies are required to obtain sufficient signal-to-noise ratios to detect objects at even moderate depths.
A need thus exists for a system which can provide an accurate and reliable image of an object embedded (particularly deeply embedded) in a very turbid medium.
The problems and deficiencies of the prior art are overcome or alleviated by the present invention. More specifically, the present invention provides for a system which can penetrate a light reflecting medium over a considerable slice (width) without requiring fast electronic devices.
The present invention provides a system for detecting a object in a light reflecting medium. The system includes a means for generating a series of discrete pulse beams in the shape of fan beams, each of which are substantially uniform in intensity, to illuminate sections of the medium.
In operation, a single pulse beam is emitted to illuminate a section of the medium. A large aperture optic collects the reflected portions of the pulse beam and focuses the reflected portions on the photocathode of a streak tube. Coupled to the streak tube is a detector which detects signals generated by the streak tube in response to the reflected portions of the pulse beam impinging on the photocathode. To obtain a volume display of the medium, the pulse beam and reflected beam are moved normal to the longitudinal axis of the pulse beam to illuminate adjacent sections of the turbid medium. A volume display of the medium is thus generated by combining the returns from adjacent sections of the medium.
The cathode on the streak tube is a thin strip behind a slit on which the illuminated strip of the scattering medium is imaged by the receiver optics. When the laser beam pulse, typically a few nanoseconds in duration, returns to the receiver from the surface of the medium the electronic sweep of the tube is initiated, so that the following time history of the returning signal spread across the lateral surface of the tube anode is then a record of the reflection from the medium itself and from any embedded bodies in the medium, including the reflection from the top surface of such objects and of the shadow below them. Because the slit cathode is long and covers the width of the medium illuminated by the fan shaped beam from the laser, the image on the anode phosphor is a wide vertical section of the medium. In addition to imaging objects entirely embedded in the medium, the invention also applies to imaging objects at a far interior surface and to obtaining a profile of interior-surface relief that may be the only way to silhouette and measure opaque irregularities or contours at that interior boundary.
The invention described herein can be useful in probing the contents of any turbid media through which light can pass, even if absorbed and scattered, as long as some return can be obtained. The items described in the following description are applicable to water probing, but there is no reason that the concept cannot be applied to the analysis of smaller volumes using very short laser pulses, picoseconds duration for example, since the streak tube can capture such time intervals.
The image on the anode can be photographed by means of a CCD camera or similar device, particularly by logarithmic area array CCD-like detectors, which is read out slowly compared to the fast duration of the returning signal. This enables one to view the phenomena on a cathode ray screen directly, or after encoding the signal, to enable one to process such images to obtain enhanced imagery through various means common to those versed in the art of enhancement, such as subtracting the mean return from the recorded section. The subsequent display of such sections can be manipulated by adding many sections together to provide a three-dimensional view of the medium and embedded objects. Such three-dimensional data sets, obtained by moving the sensor system normal to the fan beam between each exposure so that each section is from an adjacent section of the medium, provide the ability to enhance detection and reduce false alarms by rejecting images, such as relatively objects, that might not be apparent in any single section image.